In the following the invention is described by way of a system in which several sources of renewable energy are provided. These sources concern devices, which convert solar, wind, waterpower or other naturally occurring energies into electric energy. These energies are also known as “green energies”.
The following description uses solar cells as examples for the creation of green energy. Solar cells are used to convert sunlight into electric energy. Solar cells are frequently connected together on a panel in order to produce a higher voltage and power than can be obtained from a single individual solar cell. Several panels are often then connected in series with one another to form a string.
A typical photovoltaic system uses a solar cell string, which makes available a varying electric power. This power depends for example on the strength of the solar radiation (morning, midday, evening), on shading and also on the ambient temperature. This power is converted by a direct current converter into a constant intermediate circuit voltage. An inverter stage then creates an alternating voltage which can either be used immediately, for example in household devices, or can be supplied into a higher-ranking network. In either case the electrical power can be a single-phase voltage or a multiple phase voltage.
In the case of a higher power requirement it is advantageous if several solar cell strings, separate from one another, are included in a system. These strings are connected in parallel with one another and with the input to the direct current converter. These solar cell strings however are then preferable ones which exhibit a generally similar behavior at their outputs, are of similar type, are of the same size, have the same orientation in respect to the sun, and have a similar shading. Accordingly, in many cases, the output powers, especially the output voltages, are different from one another. This leads to a non-optimal use of the solar cell strings.
It has therefore been previously proposed that the individual solar cell stings each be equipped with a dedicated direct current converter. The individual direct current converters then feed a predetermined voltage to the intermediate circuit of the power converter, for example by way of a voltage bus.
Such type of arrangement is known for example from DE 101 36 147 A1.
On other hand there are also systems or installations in which the solar cell strings in respect to their type, their size, their orientation, their shading and other factors are so similar that they produce output values, for example output voltages, which are more or less identical. In this case it can be advantageous to not only connect their outputs in parallel to the direct current converters, but also to connect inputs of the direct current converters in parallel.
A direct current converter customarily has two characteristics which in many respects are disadvantageous.
First, there is an upper limit for the input power limit which cannot be exceeded without the direct current converter being damaged. Secondly, a direct current converter often has a reduced degree of effectiveness when it works with a low input power.
If one now connects the individual solar cell strings (in the following these are also in a shortened way referred to as “solar cells”) with several direct current converters, one can then for one thing see to it that the electric power is uniformly distributed to the direct current converters. For another thing one can turn off individual direct current converters if the electrical input power is too low. A direct current converter has a certain basic consumption of power even when it converts no electric power. The turning off of direct current converters therefore takes place by way of a control device in dependence of the amount of delivered electrical energy.
The control device must now “know”, in what configuration the individual direct current converters are arranged in the system. The control device may for example only turn off such direct current converter whose solar cells are connected with another direct current converter.
This information can be made available to the control mechanism by a manual input. A supervising person can for example turn a switch or enter a parameter through a keyboard. In this case however there exists to a considerable degree the danger of a faulty input, especially in the case of large photovoltaic systems made up of several hundred solar cell strings.
If the control device is supplied with faulty information this can lead to an inefficient use of the solar cell strings.